The retirement of NBC anchorman Tom Brokaw heralds the start of a generational change among the US networks' newsreaders.

Brokaw, 62, is the first of the three current network anchors to announce his departure. He will step down as the anchor of NBC Nightly News in 2004.

He will be succeeded by Brian Williams, 43, who presents a nightly cable newscast on MSNBC and CNBC.

Brokaw's rivals, Dan Rather, 74, of CBS and Peter Jennings, 63, of ABC are both renegotiating their contracts.

The anchormen of nightly news programmes hold a unique position in US journalism and are an integral part of American popular culture.

More than just newsreaders, they are considered by many Americans to be reliable guides in difficult times. For instance, all three anchors were applauded for their calming presence when covering the events of September 11.

Brokaw, Rather and Jennings differ from their younger newsreading colleagues in that they have extensive field reporting experience.

Brokaw covered Watergate, the assassinations of Robert Kennedy and Anwar Sadat and the disintegration of the former Soviet Union.

Rather reported on the shooting of John F Kennedy, the Vietnam war and the resignation of Richard Nixon.

Jennings has extensive experience as a foreign correspondent, working in Europe and putting in a seven-year stint in Beirut.

In contrast, Williams has spent most of his career behind an anchor desk.

His bosses at NBC say Williams will work on in-depth journalism and foreign reporting for the next two years.

Brokaw intends to remain active in journalism. "It doesn't mean I am going to go sit in the anchorman's rest home in a rocking chair and take soft food from January 2005," he said.